Dentures are conventionally made from plastic materials. The tooth-holding portion of a denture is generally made from a rigid polymeric material of good dimensional stability and high impact strength. The gum portion frequently causes discomfort to the denture wearer. Consequently, much of the denture art has been concerned with providing comfort as well as a good fit to the denture wearer. Attempts have been made to alleviate denture wearer discomfort by providing denture liners made of soft elastomeric materials.
The most commonly used type of polymeric material from which denture bases and denture liners have been made is acrylic plastic. Many different types of acrylate polymers, copolymers, and crosslinked systems have been utilized for denture bases and denture liners. A detailed review of such materials is presented in Restorative Dental Materials, Seventh Edition (R. C. Craig ed.), C. V. Mosby Company, (1985), pp. 458-511. In general, the method of fabrication of such materials involves the free radical polymerization of acrylate monomers.
Most denture base plastics are prepared from powder-liquid compositions comprising poly(methyl methacrylate) beads, free radical polymerization initiator such as benzoyl peroxide, and monomer liquid consisting primarily of methyl methacrylate. Polymer beads comprising higher alkyl methacrylates, such as poly(ethyl methacrylate) and poly(butyl methacrylate), are typically used in the preparation of denture liners in order to provide flexibility to the denture liner. Lower acrylate beads are typically used in the preparation of denture bases. A method of making dental articles such as denture bases and denture liners involves mixing the appropriate polymerized beads and liquid monomer in order to form a mixture, packing the mixture in prepared flasks, followed by curing the mixture by the application of heat and pressure. Cold-curing systems which utilize amine accelerators are available, but, in general, provide acrylic polymers having inferior properties. Both the heat-curing process as well as the cold-curing process are labor intensive and require lengthy time periods of several hours.
In the dental fields of denture bases and denture liners, modifications to basic polymer systems have been made in an attempt to enhance the physical properties of the denture bases and the denture liners in order to provide both a good fit and comfort to the denture wearer. However, modifications to denture base materials and denture liner materials to meet these criteria have not met with complete success.
For example, U.S. Pat. No. 4,300,886 describes a denture base made of a rigid polymethacrylate elasticized by means of diamine lengthened polyurethane. However, such a modification compromises the physical properties of the denture base.
When both unmodified and modified denture base materials failed to provide the comfort sought after by the denture wearer, denture liners were fabricated to make the denture bases more comfortable. A variety of soft elastomeric materials were formulated for use in either combination with the denture base or in integral formation with the denture base, such as those compositions disclosed in U.S. Pat. Nos. 3,339,283; 3,391,231; and 4,360,344. U.S. Pat. Nos. 3,391,231 and 3,339,283 describe a method of fabricating an artificial denture by first molding a rigid denture base and then covering it with a hydraulic cushioning means formed from a cold cure silicone rubber.
U.S. Pat. No. 4,360,344 describes a composite denture, wherein the tooth-holding portion is fabricated from a hard acrylic polymer and the mouth-engaging portion is fabricated from a soft non-hydrophilic polyurethane elastomer.
Each of the above polymers are prepared according to conventional free-radical polymerization processes. Thus, the processes once started and stopped provide materials of certain physical properties and shape which cannot be further modified without the inclusion of additional initiator. Thus, the above-described polymerization processes do not possess a "living" character.
The shaped dental articles prepared by the above-described methods are oftentimes uncomfortable for the dental patient and provide a poor fit. Dental articles also are difficult to fabricate utilizing the above-described methods.
A need therefore exists for denture bases and denture liners which are both comfortable and which provide a good fit to the denture wearer. In addition, a need exists for denture bases and denture liners which are easy to fabricate. A need also exists for denture bases and denture liners which can be gradually cured by allowing the dental practitioner to start and stop the curing process at will.
Also used in dentistry, are restoratives based upon polymerizable diacrylates or dimethacrylates containing conventional free radical initiators. Such restoratives are discussed in the aforementioned reference entitled Restorative Dental Materials, pp. 225-252. The term "extraoral restorative" as used herein refers to a restorative made and cured outside the mouth which is subsequently placed inside the mouth. The term "direct restorative" as used herein refers to a restorative placed and cured directly in the mouth such as a tooth-colored composite restorative resin useful as a tooth-filling material.
The curing of known compositions used in forming restoratives occurs via a free radical chain reaction. Once terminated, the polymerization cannot be reinitiated simply by further light exposure and hence the customary practice is for the dental practitioner to first cure the restorative fully, and then use a dental tool, such as a bur, to cut back and shape the cured restorative to the desired anatomy. This is a time-consuming process for the dental practitioner and uncomfortable for the patient if the occlusion obtained is not correct.
The concept of light curing or cure on demand is currently gaining popularity in the field of dentistry. Recently a light activated denture base resin has been marketed under the tradename Triad.RTM. by Dentsply Corporation. The materials of this system consist of a prepolymerized polymethylmethacrylate embedded in a crosslinked network of a urethane dimethacrylate. These materials are discussed in an article by E. A. Lewis, R. E. Ogle, and S. E. Sorensen, Orthodontic Applications of a New Visible Light Curing Resin System, NYS Dental Journal, March, 1986, p. 32-34. Acrylate polymerizations, both chemical and light activated, are also used extensively in direct esthetic restoratives.
The aforementioned light activated systems represent classical free radical polymerizations. The molecular weight increases rapidly with time so that the extent of polymerization and crosslinking cannot be controlled by light exposure. Although the light curing aspect is an attractive feature of these systems, the properties of these materials are not as good as the heat cured materials. According to Clinical Rsearch Associates (CRA) the best application of these materials appears to be for chair-side repairs and relining. (CRA Newsletter, Vol. 10(1), January 1986)
In all applications thus far in dentistry, the polymerization mechanism utilized has involved simple free radical kinetics. Thus, the molecular weight reaches its peak value very early during the polymerization and although extended time of reaction leads to greater conversion, this is not attendent with increased molecular weight. During the light activated polymerizations of dental materials known thus far, once the polymerization has been stopped by turning off the light source, it cannot be reinitiated by turning the light source back on.
A need therefore exists for direct and indirect restoratives, in addition to denture bases and denture liners, which can be cured gradually by allowing the dental practitioner to start and stop the curing process at will. Applicants have discovered a composition which employs iniferter technology.
The term "iniferter" or "photoiniferter" refers to a chemical compound that has a combined function of being a free radical initiator, transfer agent, and terminator, the term "iniferter" being a word formed by the underlined portions of the terms identifying these functions. The photo portion of the term indicates that the free radical polymerization is photolytically induced. This term and its use in the production of block copolymers is well known, particularly because of the work of Takayuki Otsu of the Department of Applied Chemistry, Osaka City University, Osaka, Japan. This work is discussed, for example, in an article by Otsu et al., entitled "Living Radical Polymerizations in Homogeneous Solution by Using Organic Sulfides as Photoiniferters", Polymer Bulletin, 7, 45-50 (1982) and an article by Otsu et al., entitled "Living Mono- and Biradical Polymerizations in Homogeneous System Synthesis of AB and ABA Type Block Copolymers", Polymer Bulletin, 11, 135-142 (1984) and European Patent Application No. 88303088.7, Publication date Oct. 12, 1988, Publication Number 0,286,376. Despite the rather detailed description of making block copolymers according to such disclosures, there is no disclosure of dental compositions comprising acrylic photoiniferter block polymers and free radically polymerizable monomer which are suitable for the gradual and controlled formation of dental articles such as denture bases, denture liners, and restoratives.
Copending U.S. application Ser. No. 07/356,650, filed May 19, 1989, now abandoned, which is a continuation-in-part of copending U.S. application Ser. No. 07/212,594, All et al., filed Jun. 28, 1988, now abandoned, (assigned to the assignee of the present case) discloses the use of iniferter technology in the preparation of acrylic block copolymers suitable for use in pressure-sensitive adhesive compositions.
Copending U.S. application Ser. No. 07/212,593, filed Jun. 26, 1988, Andrus Jr. et al., now abandoned, (also assigned to the assignee of the present case) discloses the use of iniferter technology in the preparation of acrylic block copolymers which can be tailored to provide optical clarity and resistance to oxidative and photochemical degradation.
The above references do not teach or suggest dental compositions comprising photoiniferter block polymer and free radically polymerizable monomer which can undergo stepwise polymerization to gradually increase the molecular weight and toughness of a dental article prepared therefrom upon controlled and intermittent exposure to ultraviolet or visible radiation by gradual polymerization and/or crosslinking.